Mast cells (MCs) are important effectors of allergic diseases, tissue injury, and protection from infections. Their development and activation are critical events in allergic inflammation and resultant diseases. Comparatively little is understood about the mechanisms that control MC development and their non-lgE-mediated activation in humans. We recently demonstrated that cysteinyl leukotrienes (cys-LTs) initiate cytokine generation by cord blood-derived human MCs (hMCs) through an interleukin (IL)-4-inducible receptor that is shared with the nucleotide uridine diphosphate. Lysophosphatidic acid (LPA), a lipid that is constitutively present in serum and tissue fluids, is unique among lipids for its behavior as a growth factor for several stromal and vascular cells, and is produced in increased quantities in malignancy, tissue injury, and inflammation. Preliminary studies indicate that hMCs express mRNA encoding all three known LPA receptors (LPA1R, LPA2R, and LPA3R, formerly designated Edg2, Edg4, and Edg7 receptors, respectively). Moreover, LPA supplementation exerts a striking, dose-dependent increase in the numbers of hMCs arising in serum-free cultures of cord blood mononuclear cells developing in the presence of stem cell factor (SCF), IL-6, and IL-10. LPA also increases the expression of c-kit, tryptase, and chymase by hMCs and enhances the development of metachromatic secretory granules. Furthermore, LPA behaves as an agonist for the exocytosis of histamine at higher doses, contrasting sharply with the activation response to cys-LTs. We hypothesize that 1) LPA facilitates basal SCF-dependent MC development and enhances it in inflammation; 2) LPARs in hMCs signal through pathways that differ from those initiated by receptors for cys-LTs, accounting for their distinct respective profiles as secretagogues; and 3) LPA is a natural ligand for MC expansion and activation and can be responsible for MC-dependent effector responses in vivo. Specific Aim 1 explores the mechanistic basis for the observed effects of LPA on MC growth and development in vitro. Specific Aim 2 contrasts the post-receptor-signaling pathways that account for differential activation responses of hMCs stimulated with LPA vs. those induced by cys-LTs. Specific Aim 3 explores the functional role of the G protein coupled receptors for LPARs in mediating MC development and MC-dependent inflammatory responses in vivo using targeted deletion of their respective genes.